CONVERSION FACTORS
SI Units
SI (le Systéme International d’Unités) units are used in many countries to express clinical laboratory and serum drug concentration data. Instead of employing units of mass (such as micrograms), the SI system uses moles (mol) to represent the amount of a substance. A molar solution contains 1 mole (the molecular weight of the substance in grams) of the solute in 1 L of solution. The following formula is used to convert units of mass to moles (mcg/mL to µmol/L or, by substitution of terms, mg/mL to mmol/L or ng/mL to nmol/L).
Micromoles per Liter
Milliequivalents
An equivalent weight of a substance is that weight which will combine with or replace 1 g of hydrogen; a milliequivalent is 1/1,000 of an equivalent weight.
Milliequivalents per Liter
Approximate Milliequivalents: Weight Conversions for Selected Ions
Valences and Atomic Weights of Selected Ions
*This appendix contains information from Appendices 1 and 2 of Anderson PO, Knoben JE, Troutman WG, et al (eds). Handbook of Clinical Drug Data, 10th ed. New York: McGraw-Hill, 2002:1053–1058, with permission.
Anion Gap
The anion gap is the concentration of plasma anions not routinely measured by laboratory screening. It is useful in the evaluation of acid–base disorders. The anion gap is greater with increased plasma concentrations of endogenous species (e.g., phosphate, sulfate, lactate, and ketoacids) or exogenous species (e.g., salicylate, penicillin, ethylene glycol, ethanol, and methanol). The formulas for calculating the anion gap are as follows:
Anion gap = (Na+ + K+)–(Cl- + 
)
or
Anion gap = Na+ –(Cl- + 
)
where the expected normal value for the first equation is 11 to 20 mmol/L and that for the second equation is 7 to 16 mmol/L. Note that there is a variation in the upper and lower limits of the normal range.
Temperature
Fahrenheit to Centigrade: (°F – 32) × 5/9 = °C
Centigrade to Fahrenheit: (°C × 9/5) + 32 = °F
Centigrade to Kelvin: °C + 273 = °K
Calories
1 calorie = 1 kilocalorie = 1,000 calories = 4.184 kilojoules (kJ)
1 kilojoule = 0.239 calories = 0.239 kilocalories = 239 calories
Weights and Measures
Metric Weight Equivalents
1 kilogram (kg) = 1,000 grams
1 gram (g) = 1,000 milligrams
1 milligram (mg) = 0.001 gram
1 microgram (mcg, µg) = 0.001 milligram
1 nanogram (ng) = 0.001 microgram
1 picogram (pg) = 0.001 nanogram
1 femtogram (fg) = 0.001 picogram
Metric Volume Equivalents
1 liter (L) = 1,000 milliliters
1 deciliter (dL) = 100 milliliters
1 milliliter (mL) = 0.001 liter
1 microliter (µL) = 0.001 milliliter
1 nanoliter (nL) = 0.001 microliter
1 picoliter (pL) = 0.001 nanoliter
1 femtoliter (fL) = 0.001 picoliter
Apothecary Weight Equivalents
1 scruple (з) = 20 grains (gr)
60 grains (gr) = 1 dram (ℨ)
8 drams (ℨ) = 1 ounce (fl ℥)
1 ounce (℥) = 480 grains (gr)
12 ounces (℥) = 1 pound (lb)
Apothecary Volume Equivalents
60 minims (m) = 1 fluidram (fl ℨ)
8 fluidrams (fl ℨ) = 1 fluid ounce (fl ℥)
1 fluid ounce (fl℥) = 480 minims (m)
16 fluid ounces (fl℥) = 1 pint (pt)
Avoirdupois Equivalents
1 ounce (oz) = 437.5 grains
16 ounces (oz) = 1 pound (lb)
Weight/Volume Equivalents
1 mg/dL = 10 mcg/mL
1 mg/dL = 1 mg%
1 ppm = 1 mg/L
Conversion Equivalents
1 gram (g) = 15.43 grains (gr)
1 grain (gr) = 64.8 milligrams (mg)
1 ounce (℥) = 31.1 grams (g)
1 ounce (oz) = 28.35 grams (g)
1 pound (lb) = 453.6 grams (g)
1 kilogram (kg) = 2.2 pounds (lb)
1 milliliter (mL) = 16.23 minims (m)
1 minim (m) = 0.06 milliliter (mL)
1 fluid ounce (fl oz) = 29.57 milliliters (mL)
1 pint (pt) = 473.2 milliliters (mL)
1 US gallon = 3.78 liters (L)
1 Can gallon = 4.55 liters (L)
0.1 milligram = 1/600 grain
0.12 milligram = 1/500 grain
0.15 milligram = 1/400 grain
0.2 milligram = 1/300 grain
0.3 milligram = 1/200 grain
0.4 milligram = 1/150 grain
0.5 milligram = 1/120 grain
0.6 milligram = 1/100 grain
0.8 milligram = 1/80 grain
1 milligram = 1/65 grain
Metric Length Conversion Equivalents
2.54 cm = 1 inch
30.48 cm = 1 foot
1 m = 3.28 feet
1.6 km = 1 mile
ANTHROPOMETRICS
Creatinine Clearance Formulas
Formulas for Estimating Creatinine Clearance in Patients With Stable Renal Function
Cockroft-Gault Formula
Adults (age 18 years and older)1:
CrCl (females) 0.85 × above value*
*Some studies suggest that the predictive accuracy of this formula for women is better without the correction factor of 0.85.
where CrCl is creatinine clearance (in mL/minute), Crs is serum creatinine (in mg/dL [or µmol/L divided by 88.4]), age is in years, and weight is in kilograms.
Children (age 1–18 years)2:
where BSA is body surface area (in m2), CrCl is creatinine clearance (in mL/minute), SCrs is serum creatinine (in mg/dL [or μmol/L divided by 88.4]), and height is in centimeters.
Formula for Estimating Creatinine Clearance From a Measured Urine Collection
where U is the concentration of creatinine in a urine specimen (in same units as P), V is the volume of urine (in mL), P is the concentration of creatinine in serum at the midpoint of the urine collection period (in same units as U), and T is the time of the urine collection period in minutes (e.g., 6 hours = 360 minutes; 24 hours = 1,440 minutes).
*The product of U × V equals the production of creatinine during the collection period and, at steady state, should equal 20 to 25 mg/kg/day for ideal body weight (IBW) in males and 15 to 20 mg/kg/day for IBW in females. If it is less than this, inadequate urine collection may have occurred, and CrCl will be underestimated.
MDRD Formula for Estimating Glomerular Filtration Rate (From the Modification of Diet in Renal Disease Study)3
Conventional calibration MDRD equation [used only with those creatinine methods that have not been recalibrated to be traceable to isotope dilution mass spectrometry (IDMS)]
For creatinine in mg/dL:
X = 186 creatinine-1.154 × age-0.203 × constant
For creatinine in µmol/L:
X = 32,788 × creatinine-1.154 × age-0.203 × constant
where X is the glomerular filtration rate (GFR), constant for white males is 1 and for females is 0.742, and constant for African Americans is 1.21. Creatinine levels in µmol/L can be converted to mg/dL by dividing by 88.4.
IDMS-Traceable MDRD Equation (Used Only With Creatinine Methods That Have Been Recalibrated to Be Traceable to IDMS)
For creatinine in mg/dL:
X = 175 × creatinine-1.154 × age-0.203 × constant
For creatinine in µmol/L:
X = 175 × (creatinine/88.4)-1.154 × age-0.203 × constant
where X is the GFR, constant for white males is 1 and for females is 0.742, and constant for African Americans is 1.21.
Ideal Body Weight
IBW is the weight expected for a nonobese person of a given height. The IBW formulas below and various life insurance tables can be used to estimate IBW. Dosing methods described in the literature may use IBW as a method in dosing obese patients.
Adults (age 18 years and older)4:
IBW (males) = 50 + (2.3 × height in inches over 5 ft)
IBW (females) = 45.5 + (2.3 × height in inches over 5 ft)
where IBW is in kilograms.
Children (age 1–18 years)2:
Under 5 ft tall:
where IBW is in kilograms and height is in centimeters.
Five feet or taller:
IBW (males) = 39 + (2.27 × height in inches over 5 ft)
IBW (females) = 42.2 + (2.27 × height in inches over 5 ft)
where IBW is in kilograms.
REFERENCES
1. Cockcroft DW, Gault MH. Prediction of creatinine clearance from serum creatinine. Nephron 1976;16:31–41.
2. Traub SI, Johnson CE. Comparison of methods of estimating creatinine clearance in children. Am J Hosp Pharm 1980;37:195–201.
3. Levey AS, Bosch JP, Lewis JB, et al. A more accurate method to estimate glomerular filtration rate from serum creatinine: A new prediction equation. Modification of Diet in Renal Disease Study Group. Ann Intern Med 1999;130:461–470.
4. Devine BJ. Gentamicin therapy. Drug Intell Clin Pharm 1974;8:650–655.